This invention relates to turbines and, in particular, to turbine rotor buckets having integral covers or tip shrouds.
Turbine blades, often referred to as buckets or airfoils, are subject to vibrational stresses that can impact engine efficiency and part life. To reduce these stresses, a number of ways of damping or limiting bucket or airfoil vibrations have been devised. One approach is to frictionally dampen certain modes of blade vibrations by interlocking the tips of covered or tip shrouded blades. To dampen vibratory stimuli and control natural frequencies, the integral covers or shrouds of the blades must maintain contact from bucket to bucket in a row. To create the requisite interlock from blade to blade, the blades are twisted during assembly. This pre-twist is in a circumferential direction as viewed along the long axis of the respective blade. During operation, centrifugal forces will cause radial growth and twisting of the bucket blades tending to open circumferential gaps between the blade tip covers. Thus, the covers must be assembled with enough compressive contact force between the respect adjacent buckets to provide residual force during operation despite the effects of centrifugal forces. The greater the interference required, the greater the required angle of rotation.
A turbine bucket integral shroud or cover configuration has thus been provided which introduces a pre-twist into the buckets by reason of an interference along contact surfaces between covers of adjacent buckets. The mating contact surfaces have steep locking angles which create a mechanical advantage which, in turn, converts a nominal tangential force into a significant axial force to cause the cover rotation or pre-twist. This pre-twist is necessary as discussed above to ensure that the covers stay coupled at speed and do not become free standing. Thus, the design of steep-angle integrally covered buckets relies on interference, created by rotating the covers to ensure continuous coupling.
When attempting to assemble rows of buckets with large amounts of interference of the type described above, manufacturing personnel have struggled to pre-rotate the final, closure blade into proper position for insertion into the notch opening. The original method of rotating the closure blade was to use a large C-clamp with a long segment of pipe to generate sufficient torque to rotate the blade into proper position. This process however, is too difficult and unsafe for commercial implementation.
A concept fixture 10 was then proposed, a schematic illustration of which is provided as FIG. 1. That fixture 10 consisted of a solid frame 12 that would fit over the notch group and apply pressure with jacking screws 14 in the axial direction. Because the fixture must be universal and because of difficulties encountered with the solid frame design in generating enough force to rotate the closure blade, a need remains for an assembly to facilitate rotation of the closure blade on completing a row of buckets during manufacture.